You need to set up a trigger for the ability. If it's an activated ability, it should be written:
"Pay 3 life: Destroy target tapped creature, it can't be regenerated."
Or:
"(0): Destroy target tapped creature, it can't be regenerated. You lose 3 life."

If it's a triggered ability, it should be:
"When Drake enters the battlefield, lose 3 life and destroy target tapped creature. It can't be regenerated."

Alternatively, the trigger could be "When you cast Drake" meaning it would still activate if drake were countered.